Evaluating clinical librarians: mixing apple merchants and orange sellers?

Clinical librarian services have been in existence for over 40 years. Although three systematic reviews have examined clinical librarian roles they continue to pose a challenge to rigorous evaluation. Difficulties can be articulated around their Population, Intervention, Comparator and Outcome (PICO) characteristics as well as within a wider evaluation framework. This feature suggests that developments within health research aimed at evaluating complex interventions may hold the key to assessing the contribution of such a variety of service models.